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Trees and People

Trees and Plants blooming out of season

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There is a saying I learned in my retail nursery trade days here in Napa: Fall is the second spring. Judging by the odd assortment of plants in bloom, it looks a lot like spring right now.

As I write this on a mid-November afternoon the temperature is about 70 degrees Fahrenheit, lawns in the neighborhood are bright green and growing, the Osmanthus shrub by my back door is in bloom and flushing with new growth, yet the deciduous trees around town are showing fall color and shedding leaves. And I have seen not one hint of frost so far this fall.

In the past two weeks, I have seen a saucer Magnolia and silver wattle Acacias coming into bloom. They usually wait until late winter here.

One of my Cymbidium orchids has two flower spikes in bloom, an event I usually see around March. An azalea I received as a gift last year and nurtured through the summer has popped into full bloom. I wasn’t expecting that until spring. The same goes for a few rhododendrons I have noticed around town.

Even the lilac shrub in my backyard is opening a few reluctant flowers. After I planted it about 10 years ago, it has consistently bloomed all at once every year and only in spring.

About the azalea, I recall someone — I do not recall who — years ago explaining that azaleas will sometimes come into partial bloom in fall when the day length is about the same as it is in spring.

That seems reasonable and interesting in light of the equal length of day and night occurring twice per year at the spring and autumn equinoxes.

If you have much interest in trees and gardening, you are probably aware that day length — photoperiod — plays a major role in the flowering cycle of many plants.

Probably the best-known example is the poinsettia. Growers have it down to a science, getting them to come into their peak of color in time for marketing just before Christmas by, in part, keeping them in total darkness for a specific number of hours per day.

Growers use various techniques to “force” azaleas and various other plants into bloom out of season, according to marketing needs.

But there are other forces at work.

For example, one reference explains that flower bud initiation (the formation of buds, not the opening of flowers) can be effected by a well-timed, short period of drought stress at a critical time in the reproductive cycle.

Curious about the off-season blooming phenomenon, I dug into some reference books and looked at some scientific papers and other resources online, hoping to find a clear explanation.

Aside from biochemistry having to do with “DNA methylation” and so forth, which is beyond my level of knowledge, the role of temperature emerged as a major player.

Much has been written on the effects of warmer fall and winter temperatures in recent years: “Confused Forsythia,” “Daffodils bloom across UK during unseasonal December weather,” “December heat tricks flowers into putting on spring display in Central Park.” And numerous articles report on a continuing pattern of spring arriving earlier as a result of warming climate.

As I have written in this space before, and as explained by University of California Cooperative Extension Farm Advisor Paul Vossen (UCCE Updates, June 2014), our region has experienced recent warm winters that have not provided the chill hours required for certain trees to get ready for their normal breaking of dormancy in spring.

In addition, Vossen wrote “chilling hours accumulated at night can be cancelled by warm daytime temperature. Warm sunny days above 60 degrees Fahrenheit reverse the portion of chilling obtained when it was cold at night.”

As a result, some trees may break dormancy erratically and bloom at abnormal times.

Locally, we can see three kinds of effects; largely a result of changing temperature patterns affecting plants differently, depending on the species:

— Erratic blooming of trees that have not received adequate chill hours.

— Extended blooming of plants that flower in summer and fall.

— Premature blooming of trees and plants normally suppressed by cold winters.

It is a changing world.

Bill Pramuk is a registered consulting arborist. Visit has website, Email questions to, or call him at 707-226-2884.

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